The little glitch that everyone from James Bond to your average Joe on the street has that stops him from reaching his true potential. They are always one of the following:
Women (my personal favourite)
Pete: Hi, my vice is womanising. I just can't stop, which means I am unable to lead a normal life.
Nick: I have alcohol problems. I can't stop drinking long enough to string three chords together.
Scott: I don't have a vice, other than womanising, gambling, drinking, smoking and kleptomania.